The field of the invention relates to idle speed control systems for motor vehicles having fuel vapor recovery systems coupled between the fuel system and engine air/fuel intake.
Idle speed control systems are known for controlling a bypass throttling device connected in parallel with the primary engine throttle. During engine idling, when the primary throttle is closed, the bypass throttle is first turned to an initial position calculated from desired idling speed and thereafter controlled by conventional feedback.
It is also known to correct for errors between the initial throttle position and the throttle position required to maintain, on average, the desired engine speed. In such adaptive systems, the initial positioning error may be learned from the feedback variable (i.e., difference between desired and actual engine speed). Thereafter, the initial throttle position is corrected by the learned error to reduce such initial positioning error.
The inventor herein has recognized several problems when conventional idle speed control systems are deployed in motor vehicles having a fuel vapor recovery system. When inducting purged air through the fuel vapor recovery system during engine idle, the bypass throttle will be reduced so that the total inducted airflow will maintain desired engine speed. The adaptive system will then learn and apply the reduced throttle position as the initial position during a subsequent idle operation. If intervening engine operation, such as cruising down a highway, recovers all the stored fuel vapors, the subsequent return to engine idle will occur without fuel vapor purging. Application of the previously learned initial throttle position may then result in engine stumble or stall.